Electronic amplifier



April 26, 1949- L. J. HEAToN-AR'MSTRONG 2,468,066

ELECTRONIC AMPLIFIER Filed Nov. 11, 1944 F/G/ HG2,

nnde Current Grid/voltage (/OJ/). /nlout Vo/tage (l2) Patented Apr. 26, 1949 ricerca ELECTRONIC AMPLIFIER Louis. John Heaton-Armstrong, London, England,

assigner, by mesne assignments, to linternational Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application November 11, i944, Serial No. 562,972 In Great Britain November 24, 1943 (Cli 179-171) 3 Claims. l

The present invention relates to radio receivers for use in radio navigational systems of the-kind employing complementary signals, for example A and N of equal strengths on respective sides of a desired path, for example an approach path or a glide path for aircraft so' as to produce on saidl path a continuous dash. orno signal.. The invention is particularly, though not solely, applicable to equipments' in which an aural indication is given, for example by means of head phones.

The complementary signalsy received [by a receiver on the path produce in the output of the receiver a current or voltage of constant amplivtude which is normally used to give a continuousdashin the earphones or a zero` indi-cation or no signal onl visual indicating apparatus. When thev receiver is off c'ourse, that is, 01T the said path there is a difference between the amplitudes of the dot arid dash signals received and the louder or greater of the two (or the smaller) indicates on which side of the path the receiver is located. When the receiver is at a great distance from the transmitters radiating the fields modulated in the complementary signal manner, for av specied degree oir course, the difieren-ce in amplitudes of the dot and dash signals is small and theu nearer the receiver is to the transmitter the greater does this difference become, but as the receiver approaches the path the difference diminishes until when on the pathl equality of amplitudes of the signals is attained. These facts will be evident from a study of the two directive 'eld distributions overlapping along the 'path since the lines of equal eld strengths are closer together nearer the origins of the distributions than further aeld. Thus the least departure from the equi-signal path the nearer the receiver approaches the origins, thev greater is the difference in amplitude between the dot and dash signals.

One object of the invention is to provide receiving arrangements in the output of which the signal changes are less violent than they would 'otherwise be when the aeroplane is near the transmitter so that the pilotdoes not have to try to keep the. aeroplane within impossibly narrow limits when about to land.

Another object is to enable the best conditions tudes is 95:16() the human ear can distinguish this ratio more easily than other. It is the ratio where the average human ear distinguishes that the equi-signal is no longer being received and that A--N or such complementary signals are being received. Electrically, however, it is desirable that when the observer hears this ratio the actual signal should be changing by a -greater ratio say :10() as interfering signals or reflections are thenl of less account. More specifically the object of this invention is to provide arrangements by which this difference in amplitude of the signals is reduced or the ratio of the amplitudes is reduced in the output of the receiver, for application to a 'pair of headphones or other indicating instrument.

According to the present invention, a radio receiver for use in a radio navigational system of the kind hereinbeiore specified comprises means for making the receiver output dot-dash signal l namely output current versus input voltage, has

a decreasing slope as the negative input grid voltage is increased whereas in the normal type of amplifier the slopes ofthe main portion of thecharacteristic curve is constant. In one practical embodiment this desired characteristic is obtained by the combination of two electron discharge devices whose individual characteristics are suitably adjusted.

The invention will be vbetter understood from the following description taken Ein conjunction with the accompanying drawings in which- Fig'. I shows diagrammatically an embodiment E of the invention in the form of a single valve ampliiier;

Fig. 2 represents the signal voltages received at a receiver;

Fig. 3 shows a typical characteristic curve of the amplifier oi Fig. l;

4 shows a preferred form of amplifier embodying the invention; and

Fig. 5 shows the characteristic curve of the amplifier shown in Fig. 4.

Referring firstly to Figs. 1 and 2, the signal leived is shown in Fig. 2 as a voltage V which is developed across resistance I 2 (Fig. 1) in the input grid circuit ci the well known variable-p amplifier Fig. l. The signals are applied as negative voltages as indicated by the arrow A in Fig. l indicating that the current flows in the resistance from il to l. The voltage V varies from one value to another in dot-dash rhythm, the variation between dot and dash amplitudes depending upon the degree of oii course and distance from the transmitting antennae. By means of the application of the signals in the manner specified the grid of the amplifier is swung negative with respect to the cathode and a signal as slriown at @l and 02 Fig. 3 is obtained, the input signal being represented respectively by ll and il Fig. 3. It will be seen that the dot-dash ratio of the signal in the output of the amplifier becomes smaller and approaches unity as the mean input level increases.

It will be observed that the signals are inverted, the stronger input dot signal being the weaker in the output, but one more ampliiication stage will reinvert the signals so that the stronger dot input signal will be represented by the stronger dot output signal. lt will be thus seen that the dasli-dt ratio of the output signal 02 is approximately 9:10 whereas that of the input signal l2 is approximately 1:2. In the case of the lower level signal input il which also has a dot-dash amplitude ratio of 1:2, the amplitude ratio of the output signal is also approximately 1:2.

An amplifier having a type of characteristic as that shown in Figure 3 and which may be varied to suit circumstances is shown in Fig. 4.

The amplifier shown in Fig. 4 comprises two electron discharge valves 3, 4 feeding into a common load resistance (i and whose inputs are taken from diierent points on an input resistance l, 2. The valve 3 has a characteristic as shown at I0 in Fig. while valve Il on account of the high resistance in its anode circuit and the smaller proportion of the input voltage applied to its grid has a characteristic as shown at l l in Fig. 5. The overall characteristic is shown at l2 Fig. 5 which is the result of the combination of curves lo and ll. Curve l2 has a more gradual bend than the curve of Fig. 3 and the shape of the bend may be varied by proportioning the values of the resistances l, 2 and 5. Curve l2 is not the mere addition or curves lil and Il. For curves l0 and il the abscissae represent grid volts. For curve l2 the abscissae represent the voltage across the resistance l, 2 and part of which only is the grid voltage oi valve il.

This nonlinear amplification of the input signals may be used in two ways:

l. Independently of the maximum amplitude value of the received signal. In this case a good automatic gain cr volume control is used on the receiver prior to the non-linear amplier, in order to keep the maximum amplitude of the signal applied to the non-linear amplier constant. lin this case the curve l2 should have a very gradual slope so that small differences in anode current are obtained for large dierences in the signal voltages applied to the input of the non-linear amplifier. In this way the listener will hear a dot-dash signal whose ratio might be say 95:10() in his telephone whereas the signal applied to the receiver might have a dot-dash ratio of :100. It has been found in practice that a ratio of :100 is best for listening to as it is the ratio which is just apparent to the ear. From the point of View of reliable operation, however, it is better to work with a large ratio say 80:100 as interfering signals, noise etc., are thus less noticeable. Thus the best listening and operating condition can be combined by adjusting the effective portion of the curve l2 for the constant level input signal of the non-linear amplier.

2. It is sometimes desirable to make the nonlinearity of the amplification small. i. e. the divergence from linear amplication small, when the amplitudes of the input signals are small and the receiver is far from the transmitter although near the approach path, and to make the amplification diverge more from linearity as the amplitudes of the signals increase, i. e. as the receiver approaches the transmitter. For example, in a particular blind approach system, when an aircraft is far away, if the aircraft departs from the approach path or course by ft. the receiver must be very sensitive to record this departure, whilst if at a few hundred yards from the transmitter for the same departure from course the indications would be too violent. One foot on course at a distance of 440 yards from the transmitter produces approximately the same amplitude ratio as 80 ft. oli course at 20 miles from the transmitter. It is, therefore, desirable to decrease the dot-dash signal amplitude differences as the dot-dash signal amplitudes increase when the receiver is nearing the transmitter. This variation of amplitude difference of the received signals with distance may be obtained by utilising known automatic gain control arrangements which are slow enough not to affect the A-N signal ratio, but allow the average amplitude of the signals applied to resistances l and 2 to increase slightly although the input signal to the receiver may greatly increase. For example, if the relative eld strength of the signal at 20 miles and 440 yards was 30:1, the automatic gain control might limit the ratio of the voltages applied to resistances l and 2 to 3:1. Thus the value of the input signal voltages to the nonlinear amplifier would move round the bend of 55 curve in Fig. 5 and the anode current change in the output of the non-linear amplier for a given voltage change at the input of the receiver becomes less the greater the said input voltage.

Whilst two embodiments only have been described, by way of example, other embodiments falling within the scope of the invention as dened in the appended claims will occur to those skilled in the art.

What is claimed is:

1. In combination in a radio receiver, an output circuit including a resistor, an amplier unit comprising a plurality of tubes connected in parallel and each having a cathode, an anode and a control grid, a common output circuit for the anodes of said tubes, connections between said grids and points on said resistor at diierent potentials, and a resistor between the anode of one of said tubes and said common output circuit.

2. In combination in a radio receiver, an output circuit for the receiver including a resistor,

an amplifier unit comprising a plurality of tubes each having a cathode, an anode and a control grid, a common output circuit for the anodes of said tubes, means for applying a different p0rtion of the Voltage drop across the resistor in said first output circuit to the control grids of different tubes, and a resistor in the anode circuit of one of said tubes.

3. An amplifier unit adapted to have an automatically Variable amplication characteristic comprising an input circuit, a plurality of tubes each having at least a cathode, an anode and a control grid, a common output circuit for the anodes of said tubes, a resistor connected between said input circuit and the grid of one of said tubes, and a resistor in the anode circuit of one of said tubes.

LOUIS JOHN HEATON-ARMSTRONG.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS OTHER REFERENCES A. P. C. application to Kramar, Serial No. 274,145, published May 25, 1943. 

